A large variety of rotor mechanisms are known in the art as exemplified by U.S. Pat. Nos. 1,426,820, 4,138,848, 4,224,016, 4,324,538, 4,406,601, 4,430,050 and 5,149,256, incorporated herein by reference. Typically, the machines comprise two or more rotors with substantially parallel axes of rotation, with each rotor comprising a cylindrical portion and one or more lobe and pit combinations. The rotors are typically located within bores of a casing with the lobe tips in close proximity or in a sealing relationship with internal surfaces of the bores at different stages of each rotary cycle, or with the lobe tips or surfaces in close proximity or sealing relationship with a surface of a pit of an adjacent rotor, depending upon a position of the rotor in the rotary cycle. As adjacent rotors rotate about their central axes in opposite directions, the lobes and pits of the adjacent rotors interengage or mesh so as to achieve movement and/or pressurization of fluid located in chambers formed temporarily between the lobes and other surfaces of the rotors during each rotary cycle. Thus, if the rotary machine is used as a compressor or pump the fluid under pressure may be caused to exit the chambers via high pressure outlets. Alternatively, such rotary machines may be used as heat or expansion engines. For example, heating of fluid within the chambers may cause an increase in pressure or expansion of the fluid within the chambers resulting in movement of the rotors about their central axes.
Heat can also be added to the compressed working fluid in a place that is external to the rotors. The added heat increases the volume and/or pressure of the working fluid to further facilitate movement of the rotors.
Over many years, efforts have been made to improve the efficiency of rotary machines by adjusting the size, shape and configuration of the rotors and their respective lobe and pit arrangements. These efforts are illustrated by numerous examples in the prior art of different rotor configurations, with rotors comprising one or multiple lobes, or with adjacent rotors in the same machine comprising different configurations. Often, such efforts have given rise to increasingly complex rotor configurations and pit/lob design principles. However, rotor designs still present a significant challenge. It can be difficult to achieve proper sealing between the surfaces of the moving rotors as well as the internal surfaces of their respective bores during each part of the rotary cycle. Sometimes, the lobes and lobe tips of adjacent rotors may not mesh completely with one another. Consequently, poor sealing between the rotors may reduce the efficiency of the machine and cause an increase in vibration or noise during operation of the machine. Moreover, inappropriate intermeshing between the rotors may increase wear and thus reduce the durability of the machine.
Thus, there remains a continuing need for rotary displacement machines that are improved compared to those of the prior art, in that they exhibit at least one improved property selected from: increased efficiency, increased durability, and reduced noise or vibration.